1. Field Of The Invention
The invention relates to a tool for measuring weight and torque experienced by a drill bit in a well drilling string while drilling.
2. Description of the Prior Art
The compressive and torque loads experienced by a bit while drilling are valuable data useful for determining the nature of geological strata encountered, controlling the direction of drilling, and optimizing the performance of the drill.
Various measure-while-drilling (MWD) tools have been developed for measuring the compressive and torque loads experienced by a drill bit while drilling. Such MWD tools typically are incorporated into a drill string immediately above the drill bit, and include one or more mandrel elements serially incorporated into the drill string by means of one or more threaded joints. (see U.S. Pat. Nos. 3,686,942 and 3,827,294). Often such tools include oil filled annuli with annular pistons for transmitting drilling fluid pressure to the data generating region of the tool (see U.S. Pat. Nos. 3,968,473 and 4,120,198).
Typically, electrical signals generated by the measuring devices of the tool, (strain gages and potentiometers) are initially processed down-hole in a data processing module incorporated into the drill string up or down string from the tool. The possessed data signals are then electromagnetically and/or acoustically telemetered to the ground surface via the drill string or drilling mud.
The primary disadvantages of existing MWD tools for measuring weight and torque on bit relate to nonlinear effects distorting the measurement response due to: (1) Hysteretic error inherent in threaded joints and (2) the unpredictability of the frictional responses of seals between movable elements of the tool.
Also, the existing MWD tools, rely heavily on the integrity of sliding pressure seals isolating the measuring devices (strain gages and potentiometers) in the measuring region from the drilling mud.
Other disadvantages of existing MWD tools relate to the absence of temperature sensors capable of providing data for eliminating temperature related effects observed by the strain gage.
Another major disadvantage of existing MWD tools for measuring weight and torque on bit is their insensitivity to small variations in actual load. In particular, the mandrel element of such tools (which generate the strain data) are designed to withstand the same load as the drill bit without significant deformation. Accordingly, strain gages sensing the deformation of such mandrels responsive to load simply can and do not generate data reflecting small variations in load.
In order to enhance the sensitivity of the detector (strain gage) response, techniques have been utilized to "amplify" the actual deformation experienced by the mandrel element (see U.S. Pat. No. 3,827,294). Other enhancement techniques contemplate serially loading one or more relatively deformable mandrels in a telescoping arrangement around a load bearing member. In such arrangements the strain gages sense deformation of the deformable mandrel to a point where the mandrel engages the load bearing element which then assumes the load. (see U.S. Pat. No. 3,827,294)
However, such existing techniques for enhancing the sensitivity of the strain gages to small variations load in such MWD tools are mechanically complex and are difficult of calibration. For example the data response of the "load responsive element" described in U.S. Pat. No. 3,827,294 depends upon the initial load impressed upon it. In telescoping arrangements, the data response is degraded by deformation of the mandrel element at the point of engagement with the load bearing element.